I have a 1956 ford f-250 with a 460 bigblock and I can't seem to get it to idle smooth the motor is a 73, I have an aftermarket roller timing chain a 600cfm holly carb the motor has been re-built and has 120psi of compression in all of the cylinders (Is this the right psi for living in the high altitude state of colorado?) I have checked for vaccuum leaks with a gasoline oil mixture and found nothing the timing is 2 degrees in advance at the crank it has plenty of power and no emissions control systems on the motor please help??

 

Straight-up is normal timing. Most cams have 4° of advance built-in to the grind.

If you're using a late ('72 on) timing chain assembly, it has 4° retardation built-in to the assembly at straight-up. Various forum reports on attempts to advance the timing using the timing gear assembly have been negative.

I'd suggest using an early ('68-71) timing gear chain assembly and setting it at straight-up.

 

the cam is a stock cam for a 73 460 is there still 4 degrees advance ground into the stock cam? I was told that ford advanced and retarded the cam through the chain not the grind in the cam is this true?

 

That's correct.

 

I have just installed the chain stright up and feel no difference in the idle??????????? ok, let me ask you this how should the negone idle? should you feel an ocasional shimmer or should you feel a constant slight rumble or should you feel nothing? I have the strangest feeling that the Idle I am getting is normal because I have checked and re-checked things too many times and have not found a problem but that does not mean there is not one.

 

With a stock cam, it should have a very smooth idle. These engines were used in the luxury cars of that era and a rough idle would have been unacceptable. Have you checked the distributor vacuum advance? Try switching from full vacuum to timed vacuum.

 

I have always had the vaccuum advance to the distributor on the ported side and not the all time vaccuum side. the cam is brand new and the stock one for a 73. do you have any other suggustions????

 

Sounds like all that's left is the Holley 600. Do you have a spare carburetor you can substitute as a test?

One way to check a Holley for idle circuit blockage is to remove both idle screws and blow the passages out with a can of compressed air or carb cleaner using that straw attachment. If that solves the problem that solution will normally be short-lived as it means there's a piece of crud floating around in the circuit and disassembly will eventually be required.

 

I have tried an edelbrock and it ran even worse than the holly I dont think that it is a fuel problem because I have tried 3 different carbs and the motor still idles the same with each carb so I dont think it is fuel related do you have anything else to recomend?

 

Do you have old or stale gas, or water in fuel? Check for vac leaks. Do the idle mixture screws do anything to help, start with about 1 to 1.5 turns out. Move the timing up to 6 or 10 btdc and see if that helps. I assume the ignition is OK, wires, cap and rotor, and plugs are all OK. If none of this helps, maybe try another known GOOD carb.
bkp

 

gas is good and I am getting an even spark to all the cylinders the plugs look good no fouling on them at all I started out with the specified turns on the idel mixture and it still runs for **** no matter how much I play with it. I have also checked for vaccuum leaks with a gasoline oil mixture an found nothing when I sprayed it on the mating surfaces. can you tell me any thing else or is there any thing else to tell it seems that I am at the end of the line?

 

It sounds like you've covered all the bases. If there's plenty of power off-idle and the only issue is at idle, I'd still still favor a vacuum leak, either from the intake manifold, carb base, distributor vaccum cannister, transmission modulator and power brake booster. Maybe go over everything again with propane in hopes that the oil/gas mixture was too thick and block off the modulator/power brake booster?

 

Does this otor have an EGR valve If so check it operation and that it is fully closing at idle. Also, as recomended advance your initial timing up to about 10 degrees BTDC.

Good lcuk,

Mark

 

As mentioned, check the vac line going to the vac brake booster if you have one. The tee fitting, the rubber hose, or the booster itself could be an unexpected vac leak.
The carb has two vac ports, one under the front bowl, and one on the pass side. Pass side port is ported vac, you probably have it going to the dist.
Next, you could have a blown power valve on the carb. Has it backfired much on you, cause that will blow it on the older Holleys. Blown power valve will be dripping gas in all the time, running very rich and thus uneven at idle, sometimes see some black puffs out the tailpipe.
Double check your firing order and plug wires.
If you can get your hands on a vac gauge, post your readings at idle and at say 1000 and at 1500 rpm.
Since you have power once it gets going (right??) it should be something minor you are just overlooking. Keep us posted.

 

the engine is as bare as they come there are no emissions controls on it and the distributor is brand new the vac line is on the proted side of the carb. the carb has not backfired on me at all and there is not an ounce of smoke that I can see out of the pipes. the truck has plenty of power infact it has so much that when I flood it from a dead stop it will roast the tires through first gear and not gain traction untill the tranny shifts into second can you recommend anything else to check????????????????